Electronic loads are used for testing and calibrating current transformers and voltage transformers, for which the collective term "measuring transformer" is generally used. Such transformers will be termed measuring transformers hereinafter. Tests are made on measuring transformers with different rated powers, rated currents and rated voltages, respectively. The loads connected thereto during testing serve to measure the errors of the measuring transformers when these transformers are subjected to loads. Due to the fact that it is necessary on the one hand to test various measuring transformers having different nominal or rated powers, currents and voltages, and since it is necessary on the other hand to measure one and the same measuring transformer at different loads, it is thus required to have correspondingly different load values to properly test the transformers.
Passive loads that are made up of passive impedance elements are known. A disadvantage of passive loads is that the impedances of the lines and of the testing or calibration instrument distort the value of the loads. It is therefore necessary to have as many types of loads as there are types of testing or calibration instruments and lines or leads. An additional disadvantage of passive loads is that they contain manually operated power current switches, which renders remote control and programming impossible. This prevents fully automatic testing of measuring transformers.
Hungarian Patent Specification No. 181,188 discloses an electronic load which is adjustable. The known electronic load of Hungarian Patent Specification No. 181,188 can be adapted to different requirements by adjustment of the impedances present between the current-to-voltage converter, the voltage-to-voltage converter and the control input of the controlled voltage or current generator, respectively. The line impedances as well as the impedances of the testing or calibration instrument are rendered ineffective by the control loops contained in this electronic load.
The rated power and the rated secondary currents and voltages, respectively, of measuring transformers are standardized. Besides, there are test specifications according to which a measuring transformer having a specific rated power must be tested at this rated power and, furthermore, at various fractions of this rated power.
With the electronic load known from said Hungarian Patent Specification, only the impedance may be adjusted. This means that a specific rated secondary power of the measuring transformer can be obtained only with one specific rated secondary current or one specific rated secondary voltage, respectively.
It is the object of the present invention to improve the known electronic load in such a manner that, with the rated power being predetermined by the impedance, the rated secondary current or the rated secondary voltage, respectively, can be selected in addition, without alteration of the impedance adjustment.